At the begining of Winter 2010 my 2001 Intrepid began having issues. When its about 25 degrees or lower the brakes either work very poorly or dont work. Initially I heard a hissing noise from the brake padeal as if I had air shocks. Usually when I park in a warm enclosed area the brakes seem to be ok, but they have a mind of their own. No ryme or reason to when they act up. I dont have this issue during the warmer months.
When was the last time you had the brake system flushed and bled?
If you haven’t in a very long time (if ever), then you most likely have water in your lines, which is freezing up, and causing the issues you are experiencing.
The other possibility is a failing master cylinder or brake booster.
The hissing noise can be symptomatic of a leaking brake booster, and a lack of braking power at low temperatures can be symptomatic of leaking seals in the master cylinder. In very low temperatures, it is possible for the seals in the master cylinder to shrink just enough to make it impossible to compress the brake fluid.
But, as BC points out, moisture in the brake fluid could be at least part of the problem also.
Since this issue is about as serious as you can get in regard to safety, you really need to immediately take the car to a mechanic for a thorough check of the entire brake system. If you don’t currently have the funds for major repairs to the brakes, then the car needs to be parked until such time as you do have the funds. This could be a matter of life or death–literally.
Thanks for your help.